Field: This invention relates generally to vertical seismic profiles (VSP), which are used to obtain information needed to perform more precise interpretations of geophysical seismic data and to obtain geophysical information beyond the limits of the well bore. More specifically, a method of constructing an inverse vertical seismic profile is disclosed.
State of the Art: One current method for performing a VSP survey involves clamping a set of three component geophones to the walls of the borehole for coupling. A repeatable surface source such as an explosive detonation is used to produce a seismic signal. The seismic wave transmitted directly to the geophones and the waves reflected from geologic horizons located below the geophones (referred to hereafter as "primary reflectances" or "primary reflected waves") are recorded on the downhole geophones. The downhole geophones are then moved to a new depth location in the well and the recording sequence is repeated. This is a long and tedious process. Many problems can occur with coupling and determining the orientation of the three component geophones.
A more efficient method of obtaining similar data is to place the seismic source in the borehole and the geophones on the surface. The profile compiled in this way is referred to as an inverse vertical seismic profile (abbr. hereinafter as IVSP)
However, measurements made by the above methods can only be performed when drilling is not taking place. It is highly desirable to be able to perform the VSP recording(s) during drilling and process the data on-site, so that the drilling engineer can make use of the information. For example, when the drill bit is about to reach a boundary between different geological strata, the drilling engineer may wish to interrupt drilling or modify the drilling conditions before penetrating into the next stratum. Also, there often may be inaccuracies in the mapping of the strata. The drilling engineer may then be unable to determine whether he has penetrated the correct stratum and has a "dry hole," or whether a deeper hole is required to penetrate the desired stratum.
Methods have been developed which can be used during drilling. These methods employ the seismic signal provided by the motion of a rotary drill bit in the borehole, or by a seismic generator attached to the drill string near the bit. Such methods are disclosed in U.S. Pat. Nos. 4,718,048 to Staron et al., 4,849,945 and 4,365,322 to Widrow, 4,207,619 and 2,933,144 to Scott, the contents of which are incorporated by reference. All of these teachings utilize sensors on the earth's surface to record the directly transmitted waves and the primary reflectances thereof which bounce to the sensors from geologic horizons below the source. Consequently, each recording must be made with the source (i.e. the drill bit) located within the limits of a single location. The length of recording time is limited to relocations of the bit during drilling of 10-15 feet at most. Thus, recording time is typically less than 20-30 minutes. The resolution of IVSPs compiled by these methods is substantially limited both by the short recording times and the relocation of the drill bit down the borehole as drilling commences.
In the Staron teaching, the drill bit's signal is recorded both at the top of the drill string and on the surface of the ground. The data recorded at the top of the drill string is used as a pilot signal that represents the source signature. It is cross correlated with the seismic signals that pass through the earth and are recorded on the surface geophones. A system believed to be similar to that of Staron is in current industrial use under the tradename TOMEX (Western Atlas Downhole Seismic Services, Rector et al., Oil & Gas Journal, pp. 55-58, June 19, 1989).
The Staron method is furthermore severely limited in many drilling situations by the ability to decipher the signal recorded at the top of the drill string. The quality of the pilot signal is adversely affected by (1) internal damping in the drill stem and the losses in the mud that attenuate the amplitude of the vibrations emitted from the drill bit; (2) resonances in the drill stem, bottomhole assembly, and derrick that deform the recorded signals, and (3) the presence of parasitic signals caused by the drill pipe hitting the sides of the borehole or by surface vibrations from other machinery (Lutz et al. 1972, Transactions Soc. Petrol. Engin. vol. 253, the contents of which are hereby incorporated by reference). Because of these problems, it is difficult to use the Staron method in highly deviated wells. Also, to date the Staron method is useful only with tricone bits, whose vertical motions permit the pilot signal to be recorded at the top of the drill string. Polycrystalline diamond bits (referred to hereinafter as "PDC" bits) cannot be used with the Staron technique, because PDC bits impact the borehole horizontally (Brett et al. 1989, Soc. of Petrol. Eng., SPE 15971, the contents of which are hereby incorporated by reference). Moreover, the Staron method produces unsatisfactory results when downhole drilling motors are employed.
The compilation of a VSP or IVSP from the detected direct and reflected waves requires considerable processing of the recorded signals. A typical method used to process a recording of a plurality of randomly produced seismic signals is a pulse coding technique called Sosie (Barbier, 1982, Pulse coding in seismology, publ. Intl. Human Resource Devel. Corp., Boston, the contents of which are hereby incorporated by reference). The continuous sequence of pulses or vibrations that are produced by a drill bit are transmitted into the earth and are effectively being convolved with the earth's reflectivity response. The Sosie technique compresses a continuous sequence of pulses into a single pulse. This method is similar to echo compression techniques used in radar and sonar detection. In the Sosie technique, the onset times of the source pulses are recorded. The observed seismogram S, containing the source pulses W and the reflectance response R of the earth, is crosscorrelated with the onset of the source (in the method of Staron, the signal which is simultaneously recorded at the top of the drill string). The crosscorrelation of the two like functions produces the autocorrelation function ACF. In the Sosie method the autocorrelation of the source signature is produced: EQU S(t)=ACF W(t)*R(t) (1)
Using a long random sequence of pulses such as those provided by drill bit motion, the source autocorrelation function reduces to an individual pulse I and correlation noise. EQU S(t)=I(t)*R(t)+noise (2)
This cross-correlation procedure is known as decoding. The seismogram from the continuous pulses is now similar to that produced from a single shot pulse (i.e., dynamite) and the earth's reflectivity function.
It is the object of this invention to overcome the limitations of previous methods. More specifically, it is an object of the invention to provide a method of seismic sensing while drilling which is not affected by relocation of the source in the borehole, permitting recording times of extended duration without loss of resolution.
It is another object of the invention to provide a method which can be used with any type of downhole seismic source and with any type of drill bit.
It is a further object of the invention to provide such a method which does not require prior knowledge of the source signature, such as is obtained by recording a signal from the top of the drill string.
It is moreover an object of the invention to provide a method of recording and processing overlapping seismic signals produced from a plurality of seismic sources placed within a borehole.
It is a further object of this invention to provide such a method permitting the interval velocities for different geological strata to be determined.
Finally, it is an object of the invention to provide means for locating the position of the source relative to the IVSP and determining when the source is about to penetrate the boundary of a geologic stratum in approximately real time.